Pipelined analog-to-digital converter having a power optimized programmable data rate

ABSTRACT

The present invention is related to a pipelined analog-to-digital converter (ADC) utilizing a power distribution scheme selectively delivering both constant and variable reference currents in selected proportions to a plurality of stages and operational transconductive amplifiers (OTAs). This permits the ADC to maintain an optimized speed over power consumption ratio over a wide data rate range. Since the invention is capable of supporting a large operating range while maintaining very low power consumption relative to the data rate, the pipelined ADC in accordance with the present invention is particularly adaptable to a large number of applications.

This application is a Continuation of U.S. application Ser. No.11/538,229, filed on Oct. 3, 2006, which is incorporated herein byreference.

FIELD OF INVENTION

The present invention is related to analog-to-digital converters (ADCs).More particularly, the present invention is directed to an ADC having apipelined architecture with an improved power distribution scheme fordata rate programmability.

BACKGROUND

An ADC is an electronic circuit that converts a continuous analogsignal, such as a voltage signal, to a discrete digital number. Due tothe popularity of consumer electronic devices, there are a large numberof applications that employ ADCs with medium resolution, such as 10bits, and medium to high data rates ranging from a few MHz to severaltens of MHz. Among these applications are wireless communicationsystems, mobile phones, video components, imaging components, local areanetwork transceivers, and the like.

Pipelined ADCs have multiple stages which successively process an analoginput signal. The principal in pipelined ADCs is to find a set ofreference voltages whose sum equals the signal sample being converted.This is realized by subtracting different reference voltages from theinput sample until the residue value becomes zero, indicating that thesum of the subtracted references equals the original signal sample. Inthe pipeline, the analog residue or remainder value may be amplified byan amplifier between the subtraction steps in order to increase accuracyprior to being fed to the next stage in the pipeline.

FIG. 1 is a high-level block diagram of a conventional 10-bit pipelinedADC. There are two basic types of components within the ADC: 1) pureanalog components which include a reference voltage generator, a biascurrent generator, a track and hold (T&H) stage, front end pipelinestages (1 through 4) and back end pipeline stages (5 through 8); and 2)digital and mixed analog/digital components including a phasesgenerator, a 2 bit ADC flash unit, delay lines, and a Redundant SignedDigital (RSD) error correction unit.

In the pipelined architecture shown in FIG. 1, the ADC has nine stageswhich process the analog input signal. However, the number of stages ina pipelined architecture can be any number depending on the desiredresolution of the ADC. The greater the number of stages in an ADC thehigher the resolution. From the left side of FIG. 1, the analog inputsignal first enters the (T&H) stage. Subsequent pipeline stages 1through 8 process the T&H output and drive a 2 bit ADC on the right. Ineach of the nine stages, an operational transconductive amplifier (OTA)is the active consuming analog cell. As an example, the ADC shown inFIG. 1 uses an external s-bits bus (speed<s−1:0>), where s may be anynumber, in order to program the ADC with a specific amount of activecurrent that is proportional to the targeted data rate of the ADC.

Referring to FIG. 2, a block diagram of a single pipeline stage isshown. The single pipeline stage includes a sample and hold (S/H) unitfor providing a constant analog signal, an ADC, a digital-to-analogconverter (DAC), a summer and an OTA. The input V_(j) is the samplenumber _(j) coming out from the previous pipeline stage as a new inputto the present stage. It is sampled and held, and also quantized with alow resolution 2 bit ADC. The resulting digital word D_(j) <1:0> isconverted back to analog using a 2 bit DAC and subtracted from theoriginal held value to create a residue. The residue is then amplifiedby G_(j) to generate an output voltage V_(j+1) to the next pipelinestage given by Equation (1) as follows:

V _(j+1) =Gj·(V _(j) −Vj ^(DA)(D _(j))).  Equation (1)

When considering the switched capacitor implementation, the S/H functionas well as the DAC, the voltage subtraction and the residueamplification may be performed by a single operation known by thoseskilled in the art as a multiplying DAC or MDAC. The MDAC performs theoperation in a period that is set by half a clock period, due to theswitched capacitor circuit implementation. During a first half clockperiod the sample value is stored in a first capacitor. During a secondhalf clock period the sample value is amplified by an OTA and multipliedby a gain value.

Since the 2 bit ADC-DAC implementation has a very low resolution, it isdesigned to provide a very high speed response. A limiting element ofthe pipeline stage, in terms of speed, is the OTA, which performs andprovides the V_(j+1) output. Setting the gain G_(j) with a high enoughresolution not to degrade the final ADC converter resolution, results inhigh power consumption. Additionally, the overall speed of the OTA isdirectly dependent upon the power consumption. The more power that isprovided to the OTA, the faster the speed of the OTA and the conversiondata rate of the ADC device. However, the increased speed of the OTAresults in higher power consumption of the ADC. This is a significantdrawback, particularly for wireless devices and other consumerelectronic devices which rely upon a battery as a power source.

Accordingly, what is needed is a versatile pipelined ADC which canoperate at a desired resolution over a wide operating range without thedrawback of high power consumption of current pipelined ADCs. Moreover,an ADC that is independent of circuit process variations for providing areliable resolution is desirable.

SUMMARY

The present invention is related to a pipelined ADC utilizing a powerdistribution scheme selectively delivering both constant and variablereference currents in selected proportions to a plurality of stages andOTAs. This permits the ADC to maintain an optimized speed over powerconsumption ratio over a wide data rate range. Since the invention iscapable of supporting a large operating range while maintaining very lowpower consumption relative to the data rate, the pipelined ADC inaccordance with the present invention is particularly adaptable to alarge number of applications.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding of the invention may be had from thefollowing description, given by way of example and to be understood inconjunction with the accompanying drawings wherein:

FIG. 1 is a block diagram of a conventional pipelined ADC;

FIG. 2 is a block diagram of a conventional single pipeline stage;

FIG. 3 is a block diagram of a pipelined ADC in accordance with thepresent invention;

FIG. 4 is a block diagram of the current programming scheme per pipelinestage in accordance with the present invention;

FIG. 5 is a schematic diagram of a fully differential folded cascode OTAin accordance with the present invention; and

FIG. 6 is a process for providing analog to digital conversion having aselectable data rate in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention would be described with reference to the drawingswherein like numerals represent similar elements throughout. Forpurposes of describing the present invention, the phrase low, medium, orhigh voltage levels may be used. It will be appreciated that the words“low”, “medium”, and “high” are relative terms and not necessarily afixed voltage. Accordingly, the phrase low, medium, or high voltagelevel may be any voltage and may vary, for example, based on theprocessing technology and/or the material in which an electronic deviceis implemented. The word “level” may represent a fixed voltage or avoltage range, as desired. Moreover, predetermined voltage levels in thedescription forthcoming can be any voltage level and may be dependent onthe design, structure, and materials used to configure a circuitelement. A node, a voltage at a node, or a current at a node may be usedinterchangeably and a load capacitance may be a parasitic capacitance inthe description forthcoming. It should be understood by those of skillin the art that the equations and variables set forth below areexemplary and should not be understood to limit the invention.

A block diagram of a pipelined ADC in accordance with the presentinvention is shown in FIG. 3. The ADC 200 includes a bias current unit201, a track and hold (T&H) unit 204, a plurality of pipeline stagesincluding pipeline stages 1 through 4 206 ⁰⁻³, pipeline stages 5 through8 206 ⁴⁻⁷, and a flash ADC unit 216. Although stages 1 through 4 and 5through 8 are shown as separated into two distinct groups this is onlyfor simplicity of explanation. Each stage may be provided with anindependent reference current level by bias current unit 201, as will beexplained in detail hereinafter, and an independent operating point. TheADC circuit 200 may also have any number of pipeline stages dependingupon the desired resolution of the ADC circuit 200. It should beunderstood by those of skill in the art that pipelined ADCs may comprisemany other components to support the various resolutions of the ADCcircuit 200. However, for simplicity, only those portions of thepipelined ADC in accordance with the present invention are described indetail hereinafter.

The bias current unit 201 includes a reference current generator 202which may be a proportional to the absolute temperature (PTAT) referencecurrent generator, a constant mode current generator 205 and an activespeed current generator 210. The reference current generator 202generates and provides reference current, which is proportional totemperature in the case of a PTAT reference current, to the constantmode current generator 205 and the active speed current generator 210. APTAT reference current generator 202 may be desirable in order to makethe present invention insensitive to temperature effects. Considering aprogrammed ADC speed, the use of a PTAT reference current generatorwould maintain the desired OTA bandwidth constant over any temperaturevariations providing compliance for any desired operational temperaturerange for the present invention.

The constant mode current generator 205 generates constant mode biascurrent ic to the individual pipeline stages 206 ⁰⁻⁷. The active speedcurrent generator 210 selectively generates variable or programmablebias current ia to the individual pipeline stages 206 ⁰⁻⁷. In general,the bias current unit 201 generates and distributes current to the T&Hunit 204 and the individual pipeline stages 206 ⁰⁻⁷ via 2 separatechannels; a first channel 212 for conducting constant mode current icand a second channel 214 for conducting variable current ia.

The first channel 212 comprises a bus of nine lines; one line for eachactive component; (i.e. the T&H unit 204 and the pipeline stages 206⁰⁻⁷). Accordingly, constant mode current ic is distributed via one ofthe lines to the T&H unit 204 and the pipeline stages 206 ⁰⁻⁷. Theconstant mode current ic, generated by the constant mode currentgenerator 205, is a fixed current which depends on the mode of operationof the ADC.

The second channel 214 comprises a bus of nine lines; one line for eachactive component. Accordingly, active speed current ia is distributedvia one of the lines to the T&H unit 204 and the pipeline stages 206⁰⁻⁷. The active speed current ia may be used to provide increasedflexibility and greater bandwidth to the T&H unit 204, as desired. Themode<1:0> input to bias current unit 201 may be used to selectivelycontrol the operation of the currents ic and ia.

The active speed current ia is proportional to the desired speed<s−1:0>, where s may be any number, which may offer 2^(S) codes ofprogrammability. Although up to two bits of programmability is shown inFIG. 3, s may be any value used to provide higher desired levels ofanalog to digital resolution, as desired.

Still referring to FIG. 3, the T&H unit 204 samples an analog inputsignal, maintains that signal level for a predetermined duration, andpasses the signal to the pipelined stages 206 ⁰⁻⁷ for sequentialprocessing of the signal. The final pipeline stage transfers the lastanalog signal to the flash ADC unit 216 for quantization of the pipelineremaining residue. In accordance with the present invention, the biascurrent unit 201 selectively delivers constant mode and active speedcurrent ic, ia, respectively, to each of the active components 204 and206 ⁰⁻⁷. Since each stage may be selectively provided with a constantmode and an adaptive active speed currents ic, ia respectively, thepipelined ADC 200 can be programmed to provide any desired data ratewhile minimizing power requirements. Power is conserved since the use ofan unnecessarily high active current may be prevented by independentlyadjusting the current level of each pipelined stage.

Referring to FIG. 4, a schematic diagram of the bias current unit 201which generates and selectively outputs current for each pipelined stage206 ⁰⁻⁷ in accordance with the present invention is provided, The biascurrent unit 201 includes the reference current generator 202, which maybe a PTAT reference current to compensate for any temperature effects,and a plurality of bias stages 250 ⁰⁻⁸; one for each pipelined stage 206⁰⁻⁷ and the T&H unit 204.

The constant mode current generators 205 ⁰⁻⁸ and the active speedcurrent generators 210 ⁰⁻⁸ may comprise separate and distinct componentsas shown in FIG. 4. In FIG. 4, each current generator may comprise aportion of a bias stage 250. The reference current generator 202distributes a reference current ib_(i) [i=0 . . . 8] to each of theplurality bias stages 250 ⁰⁻⁸ that provide current to each pipelinedstage 206 ⁰⁻⁷ and the T&H unit 204. Although nine bias stages are shownin FIG. 4, any number of bias stages may be used depending upon thedesired number of pipelined stages. Each bias stage 250 ⁰⁻⁸ receivesib_(i), which is used as a current reference iref as shown by 203 ⁰⁻⁸,and transmits both constant mode current ic_(i) and active speed currentia_(i) to its corresponding pipeline stage 206 ⁰⁻⁷ and T&H unit 204.

The transfer function of the local bias stage number i is given asfollows:

ic _(i) =c·iref;  Equation (2)

ia _(i)=(a _(i)+1)·iref  Equation (3)

where iref is the reference current provided by unit 202, c is aconstant integer dependent upon the desired OTA operating point, anda_(i) is a programmable or variable number, such as an integer,dependent upon the desired variable program speed <s−1:0>. Since a_(i)is a variable, the ADC can be used to provide an active current that isproportional to any desired data or conversion rate.

Table 1 provides purely as an example a case where s=2.

TABLE 1 Programmed Speed Current ia₁ speed<1> speed<0> a₁ (unit in iref)0 0 0 1 0 1 1 2 1 0 2 3 1 1 3 4

Since a_(i) is a variable, each OTA in the pipeline ADC 200 may beprogrammed using any amount of current ia_(i) as given in Equation (3)and shown in Table 1. Since the OTA in accordance with the presentinvention may be programmable, the OTA can provide a bandwidth largeenough to be compatible with any desired ADC data rate.

Each pipelined stage 206 ⁰⁻⁷ in accordance with the present inventionmay be configured similar to the general configuration shown in FIG. 2.However, the present invention includes an OTA 300 which is adapted totake advantage of the two-part current distribution scheme shown inFIGS. 3 and 4 for adjusting the power consumption to any desired datarate. With the power consumption adapted to the desired ADC data rate,this data rate is considered as the maximum data rate (MDR) of the ADC.Referring to FIG. 5, a transistor level schematic of an OTA 300 inaccordance with the present invention is shown. Although, there are aplurality of bias stages 250 ⁰⁻⁸ in a pipelined ADC having acorresponding plurality of OTAs 300, only one OTA 300 is shown forsimplicity.

The OTA 300 is a fully differential folded cascode topology using ann-type metal-oxide semiconductor (NMOS) transistor input pair. As thosethat are skilled in the art would realize, the p-type metal-oxidesemiconductor (PMOS) type transistor input pair version could also beconsidered. The output of the OTA 300 may be loaded by single-ended loadcapacitors C1.

The Common Mode FeedBack box (CMFB) provides regulation of the commonmode output voltage ensuring that either vop=von=vcm when vip=vin at theinput of the OTA 300 or (vop+von)/2=vcm for all other cases. The twoseparate input currents, the active speed current ia at node 302 and theconstant mode current ic at node 304 are received from a respective biasstage 250 ⁰⁻⁸. The constant mode current ic is derived from Equation (2)and is used to set the internal cascode voltage nodes vcas 304, 306, and308 in order to maximize the dynamic output range of the OTA 300. Theactive speed current ia is derived from Equation (3) and is used toprovide a desirable unity gain bandwidth for OTA 300.

A metric used to measure the performance of circuits 200 and 300 is theratio of the maximum data rate (MDR) to the power dissipation, sometimesreferred to as the figure of merit (FOM) of an ADC device, given byEquation (4) as follows:

$\begin{matrix}{{F\; O\; M} = {\frac{M\; D\; R}{POWER}.}} & {{Equation}\mspace{14mu} (4)}\end{matrix}$

The MDR of ADC 200 is given by Equation (5) as follows:

$\begin{matrix}{{M\; D\; R} \approx {A \cdot {\frac{k*{\left( {a_{i} + 1} \right) \cdot {iref}}}{Cl}.}}} & {{Equation}\mspace{14mu} (5)}\end{matrix}$

In Equation (5), A is a fixed parameter dependent upon a desiredprocessing technology and the desired resolution (i.e. number of bits)of an ADC, and k is the current amplification factor of OTA 300.Therefore, MDR is directly proportional to the current amplificationfactor k, the reference current iref, and the local active currentvariable a_(i) for each stage as set forth in Equation (3) and shown inFIG. 4. MDR is inversely proportional to the equivalent capacitance,including any parasitic capacitances, Cl seen at the output of circuit300.

In Equation (4), the dissipated POWER metric is calculated by summingthe current consumed by each pipeline stage 206 ⁰⁻⁷ and T&H unit 204.Since each pipeline stage 206 ⁰⁻⁷ and T&H unit 204 may include an OTAsimilar to 300, the total dissipated POWER is given by Equation (6) asfollows:

POWER=1000*P*idd*V _(DD) (in mW).  Equation (6)

In Equation (6), P is the total number of OTA circuits 300 used in ADC200, V_(DD) is the supply voltage level, and current idd is the supplycurrent level at node 310 for each stage given by Equation (7) asfollows:

idd=ic _(i)+(4k+2)*ia _(i).  Equation (7)

In Equation (7), k is the current amplification factor of OTA 300,ic_(i) is the constant mode current at node 304, and ia_(i) is theactive speed current at node 302.

Substituting Equations (2) and (3) into Equation (7) gives Equation (8)as follows:

idd=(c+2(2k+l)(a _(i) +l))*iref.  Equation (8)

In Equation (8), the variables c, k, a_(i), and iref are the same asthose provided in Equations (2), (3) and (7) above. Since the supplycurrent idd of each OTA stage 300 is dependent upon the programmablevariable a_(i), both the OTA unity gain bandwidth and power consumptionmay be optimized for OTA 300 and ADC 200, respectively.

Substituting Equations (5), (6), and (8) into Equation (4) and assuming2(2k+1)>>c, the FOM may be given by Equation (9) as follows:

$\begin{matrix}{{F\; O\; M} \cong \frac{A \cdot k}{{2000 \cdot P \cdot {Vdd} \cdot \left( {{2k} + 1} \right) \cdot C}\; 1}} & {{Equation}\mspace{14mu} (9)}\end{matrix}$

In Equation (9), c, a_(i) and iref have been removed from the expressionfor the FOM. The ADC 200 has been made independent of the programmeddata rate and corresponding current consumption which results in ADC 200performing at a desirably optimized constant speed over power ratio.

FIG. 6 is a process 400 for providing steps for analog to digitalconversion having a selectable data rate and optimized power dissipationin accordance with the present invention. An ADC is RESET and the MODEand SPEED are preset to a default value providing a default bias currentsetting which includes a set of constant bias current and a set of(preset) variable or active bias current (step 402). A variable analogsignal level is sampled (step 404) to provide a constant analog signal.The constant analog signal level is provided to an analog to digitalconverter for producing a first digital output number which is thenconverted back to a converted analog signal via a digital to analogconverter (step 406). A first analog remainder is provided bysubtracting the second analog signal from the sampled analog signallevel (step 408). The first analog remainder signal is used to produce asecond digital output number and a second analog remainder (step 410).The first and second digital output numbers are summed to produce adigital output signal (step 412). Changing the SPEED programming togenerate a new set of variable bias current, for producing the digitaloutput signal to the desired data rate while dissipating a minimumamount of power (step 414) may then be performed.

Although the features and elements of the present invention aredescribed in the preferred embodiments in particular combinations, eachfeature or element can be used alone without the other features andelements of the preferred embodiments or in various combinations with orwithout other features and elements of the present invention. Thepresent invention may be implemented in a computer program or firmwaretangibly embodied in a computer-readable storage medium having machinereadable instructions for execution by a machine, a processor, and/orany general purpose computer for use with or by any non-volatile memorydevice. Suitable processors include, by way of example, both general andspecial purpose processors.

Typically, a processor will receive instructions and data from a readonly memory (ROM), a RAM, and/or a storage device having stored softwareor firmware. Storage devices suitable for embodying computer programinstructions and data include all forms of non-volatile memory,including by way of example semiconductor memory devices, read onlymemories (ROMs), magnetic media such as internal hard disks andremovable disks, magneto-optical media, and optical media such as CD-ROMdisks and digital versatile disks (DVDs). Types of hardware components,processors, or machines which may be used by or in conjunction with thepresent invention include Application Specific Integrated Circuits(ASICs), Field Programmable Gate Arrays (FPGAs), microprocessors, or anyintegrated circuit.

1. An analog to digital converter, comprising: an input unit to receivea variable analog input signal and to produce a constant analog signal;a first pipeline stage to receive the constant analog signal and toproduce a first digital output number and a first analog remaindersignal; at least one subsequent pipeline stage to receive the firstanalog remainder signal and to produce a second digital output numberand a second analog remainder; a summing unit to sum the first andsecond digital output numbers to generate a digital output signal; and abias current generation unit, comprising: a constant bias currentgenerator, separately coupled to said input unit and each pipelinestage, to generate a constant current; a variable bias currentgenerator, separately coupled to said input unit and each pipelinestage, to generate a variable current that is related to the selectabledata rate; and and a reference current generator, coupled to theconstant bias current generator and the variable bias current generator,for generating a reference current.
 2. The converter of claim 1, whereinthe first digital output number is delayed by one half of a clock periodcompared to said second digital output number.
 3. The converter of claim1, wherein the input unit further comprises a sampler for sampling thevariable analog input signal to produce said constant analog signal. 4.The converter of claim 1, wherein said constant bias generator utilizessaid reference current to generate said constant current.
 5. Theconverter of claim 4, wherein said constant current is a fixed currentdependent on the mode of operation for the Analog to Digital Converter.6. The converter of claim 1, wherein said variable bias currentgenerator utilizes said reference current to generate said variablecurrent.
 7. The converter of claim 1, wherein said variable bias currentgenerator selectively generates said variable current for the firstpipeline stage and the subsequent pipeline stage.
 8. The converter ofclaim 1, wherein the reference current generator is to generate areference current that is proportional to an absolute temperature (PTAT)reference current.
 9. The converter of claim 1, wherein the bias currentgeneration unit further comprises a plurality of bias stages, one biasstage coupled to said input unit and each pipeline stage.
 10. Theconverter of claim 1, wherein the variable bias current generatorgenerates a different variable current for each pipeline stage.
 11. Theconverter of claim 1, wherein the variable bias current is proportionalto a programmable variable.
 12. The converter of claim 10, wherein theprogrammable variable is dependent upon a program speed variable. 13.The converter of claim 1, wherein the summing unit is a final pipelinestage that quantizes the remaining residue of the pipeline.
 14. Theconverter of claim 13, wherein the quantized remaining residue of thepipeline is optimized for power by a figure of metric relationship. 15.An analog to digital converter, comprising: an input unit to receive avariable analog input signal and to produce a constant analog signal,the input unit including a sampler to sample the variable analog inputsignal to produce the constant analog signal; a first pipeline stage toreceive the constant analog signal and to produce a first digital outputnumber and a first analog remainder signal; at least one subsequentpipeline stage to receive the first analog remainder signal and toproduce a second digital output number and a second analog remainder,wherein the first digital output number is delayed by one half of aclock period compared to said second digital output number; a summingunit to sum the first and second digital output numbers to generate adigital output signal; and a bias current generation unit, comprising: aconstant bias current generator, separately coupled to said input unitand each pipeline stage, to generate a constant current; a variable biascurrent generator, separately coupled to said input unit and eachpipeline stage, to generate a variable current that is related to theselectable data rate; and a reference current generator, coupled to theconstant bias current generator and the variable bias current generator,for generating a reference current.
 16. The converter of claim 15,wherein the reference current generator is to generate a referencecurrent that is proportional to an absolute temperature (PTAT) referencecurrent.
 17. The converter of claim 15, wherein the bias currentgeneration unit further comprises a plurality of bias stages, one biasstage coupled to said input unit and each pipeline stage.
 18. Theconverter of claim 15, wherein the variable bias current generatorgenerates a different variable current for each pipeline stage.
 19. Theconverter of claim 15, wherein the variable bias current is proportionalto a programmable variable.
 20. The converter of claim 19, wherein theprogrammable variable is dependent upon a program speed variable. 22.The converter of claim 15, wherein said constant current is a fixedcurrent dependent on the mode of operation for the Analog to DigitalConverter.
 23. The converter of claim 15, wherein the summing unit is afinal pipeline stage that quantizes the remaining residue of thepipeline.
 24. The converter of claim 23, wherein the quantized remainingresidue of the pipeline is optimized for power by a figure of metricrelationship.
 25. An analog to digital converter, comprising: an inputunit to receive a variable analog input signal and to produce a constantanalog signal, the input unit including a sampler to sample the variableanalog input signal to produce the constant analog signal; a firstpipeline stage to receive the constant analog signal and to produce afirst digital output number and a first analog remainder signal; asumming unit to sum the first and second digital output numbers togenerate a digital output signal; and a bias current generation unit,comprising: a constant bias current generator, separately coupled tosaid input unit and each pipeline stage, to generate a constant current;a variable bias current generator, separately coupled to said input unitand each pipeline stage, to generate a variable current that is relatedto the selectable data rate, wherein said variable bias currentgenerator generates a different variable current for each pipelinestage; and a reference current generator, coupled to the constant biascurrent generator and the variable bias current generator, forgenerating a reference current.
 26. The apparatus of claim 25, whereinthe programmable variable is dependent upon a program speed variable.27. An analog to digital converter, comprising: an input unit to receivea variable analog input signal and to produce a constant analog signal,the input unit including a sampler to sample the variable analog inputsignal to produce the constant analog signal; a first pipeline stage toreceive the constant analog signal and to produce a first digital outputnumber and a first analog remainder signal; a summing unit to sum thefirst and second digital output numbers to generate a digital outputsignal; and a bias current generation unit, comprising: a constant biascurrent generator, separately coupled to said input unit and eachpipeline stage, to generate a constant current; a variable bias currentgenerator, separately coupled to said input unit and each pipelinestage, to generate a variable current that is related to the selectabledata rate, wherein said variable current is proportional to aprogrammable variable; and a reference current generator, coupled to theconstant bias current generator and the variable bias current generator,for generating a reference current.
 28. The apparatus of claim 27,wherein the programmable variable is dependent upon a program speedvariable.
 29. A method for converting an analog input signal into adigital output signal, said method operating at a selectable data rate,comprising: setting a speed variable to provide a constant current and avariable current; choosing said speed variable to optimize powerconsumption. sampling a variable analog signal level; producing a firstdigital output number and a first analog remainder signal related to thesampled variable analog signal level; providing the first analogremainder signal to produce a second digital output number and a secondanalog remainder, wherein said first digital output number is delayedcompared to said second digital output number; summing said first andsecond digital output numbers to generate said digital output signal;and changing the speed variable to produce the first digital outputnumber and the second digital output number at the selectable data rate.30. The method of claim 29, comprising using a reference current togenerate said constant current and said variable current.
 31. The methodof claim 29, comprising delaying the first digital output number by onehalf of a clock period compared to said second digital output number.32. The method of claim 29, comprising resetting a converter andpresetting a mode.
 33. The method of claim 29, wherein choosing saidspeed variable involves maximizing the figure of merit for an analog todigital converter.